In the future, the amount of nitrogen oxide gases emitted from an automobile will have to be reduced to levels of about 100 p.p.m. Also, most carbon monoxide and hydrocarbons present in the exhaust gas will have to be eliminated. The contemplated system for treating these exhaust contaminants includes a multiple-stage catalytic converter, e.g., a two-stage catalytic converter. The first stage or reactor treats, under reducing conditions, exhaust gases directly coming from the exhaust system. In this first reactor, the nitrogen oxide (NO.sub.x) gases in the exhaust gas react with hydrocarbons and carbon monoxide in the exhaust gas. Just prior to, or in the second stage or reactor, oxygen is injected into the exhaust stream. Oxidation occurs in this second stage where the hydrocarbons and carbon monoxide react with the oxygen. External oxygen may also be injected into the first stage at engine start up in order to meet federal emission standards for carbon monoxide and hydrocarbons. When the engine is warmed up, oxygen injection into the first stage is terminated. Each stage of the two-stage converter contains different types of catalyst. This type of converter requires that the internal combustion engine must be operated at fuel-rich conditions, insuring the presence of sufficient carbon monoxide and hydrogen to react with the nitrogen oxide gases.
The catalyst in the first stage is called the NO.sub.x catalyst, and it promotes the reduction of nitrogen oxides. Several different and sometimes competing reactions can and do occur as the exhaust gas flows over the NO.sub.x catalyst. The most important reactions are: EQU NO.sub.x + CO.fwdarw.N.sub.2 + CO.sub.2 ( 1) EQU no.sub.x + H.sub.2 .fwdarw.NH.sub.3 + N.sub.2 + H.sub.2 0 (2) EQU 2co + o.sub.2 .fwdarw.2co.sub.2 ( 3) EQU 2h.sub.2 + o.sub.2 .fwdarw.2h.sub.2 0 (4)
reaction No. 1 is the reaction responsible for eliminating NO.sub.x as an exhaust pollutant. Advantageously, the NO.sub.x catalyst favors this reaction rather than Reactions Nos. 3 and 4, and preferably, over a wide range of temperatures, for example, from 700.degree. to 1,700.degree. F. Reaction No. 2 is undesirable because ammonia formed in the first stage of the converter will be converted to nitrogen oxide in the second stage, defeating at least in part the purpose of the converter. Consequently, a desirable NO.sub.x catalyst will not promote Reaction No. 2 or will minimize formation of NH.sub.3 in favor of N.sub.2. (Reduction to NH.sub.3 occurs most frequently at low temperatures.) Reactions Nos. 3 and 4 are undesirable in the first stage because carbon monoxide and hydrogen are oxidized. The NO.sub.x catalyst requires that some carbon monoxide be present in the exhaust gas. However, there is also some oxygen present in the exhaust gas, which oxygen reacts with carbon monoxide using, in the first stage, the carbon monoxide least advantageously. The preferred NO.sub.x catalyst will selectively promote the reaction of carbon monoxide with NO.sub.x in the presence of small amounts of oxygen (up to about 1%).
It is believed that an effective NO.sub.x catalyst should have the following characteristics:
1. It must be effective at temperatures ranging from 700.degree. to 1,700.degree. F. At the low end of this temperature range, it must remove or reduce at least 50% of the NO.sub.x in the exhaust gas. At the high end of this range, it must remove or reduce about 90% of the NO.sub.x in the exhaust gas.
2. It must be stable at the higher temperatures under alternating oxidizing and reducing conditions, that is, the catalyst must retain a high activity.
3. It must be effective at fuel-air ratios close to stoichiometric [carbon-monoxide-to-oxygen (CO/O.sub.2) ratios near 2 and CO concentrations about 1%]. This conserves on fuel.
4. It must resist sulfur poisoning, since sulfur, at least in small amounts, is present in most grades of gasoline.
5. It must minimize ammonia formation at low operating temperatures.
6. Preferably, it should be capable of promoting at low temperatures (700.degree. F. and lower) the oxidation of carbon monoxide. This is desirable in order to pass the federal cold start test procedure.
There have now been found an improved NO.sub.x catalyst and a process for treating exhaust gases, which process employs that catalyst.